1. Field of the Invention
The present invention relates to a building block type exchanger in which a plurality of exchange modules are connected through an exchange module connector, and more particularly, to a data transmission and a transmission path setting among the exchange modules in such a building block type exchanger.
2. Description of the Background Art
A building block type exchanger is an exchanger formed by a plurality of exchange modules as blocks, where each exchange module is connected with external subscriber lines or relay transmission lines and the exchange modules are mutually connected in the exchanger by an exchange module connector having a cross-connect function for directing data transmitted from one input path to a desired output path. The examples of the exchange module connector to be used here include an STM (Synchronous Transfer Mode) exchange module connector and ATM (Asynchronous Transfer Mode) exchange module connector.
Here, each exchange module itself is a small unit exchanger, and the reason for building a larger exchanger (building block type exchanger) by accumulating a plurality of smaller unit exchangers as blocks is to unify the architecture of the exchangers regardless of their sizes such that the exchanger of any desired size can be constructed by using the same type of unit exchangers.
For such a building block type exchanger, there are two conventionally known methods for the transmission of the control data among the exchange modules in the exchanger.
One method is to use the STM exchange module connector and the STM paths exclusively allocated for the control data transmission which are to be provided between each exchange module and the STM exchange module connector.
The other method is to use the ATM exchange module connector in which the data are transmitted in units cells, where each cell has a fixed cell length and a header indicating the destination of each cell.
However, these two conventionally known methods for transmission of the control data among the exchange modules in the exchanger are associated with the problems.
Namely, the method using the STM exchange module connector has the problem in that, in order to provide the STM path between the STM exchange module connector and each one of the exchange modules in the exchanger, as many as (a number of exchange modules in the exchanger).times.(a number of STM paths required for the transmission of the control data between any two exchange modules) of the STM paths exclusively allocated for the control data transmission must be provided in the exchanger, while the bandwidth required for the transmission of the control data between any two exchange modules is actually less than the bandwidth of a single STM path, so that the utilization efficiency of all these STM paths exclusively allocated for the control data transmission as a whole becomes quite low.
Moreover, as the number of exchange modules exchanger increases, some of these STM paths becomes actually wasteful because there is a limit to a number of exchange modules with which each exchange module can exchange the control data simultaneously due to the limited capacity of the processor.
In addition, as the number of exchange modules in the exchanger increases, a number of I/O interfaces required to be provided on each exchange module also increases, and this in turn causes a complication of the hardware and the software of the exchanger as well as an increase of the cost of the exchanger.
Furthermore, the cost required for the control data transmission lines increases in proportion to (burstyness of the control data transmission).times.(a number of exchange modules in the exchanger), and this fact is potentially problematic as the burstyness of the control data transmission is expected to increase in future because the diversity of the services which are expected to be accommodated. This problem is more troublesome because the cost of the transmission line is generally higher than cost of the exchanger itself. The expected increase of the burstyness also reduces the utilization efficiency of the control data transmission lines, so that this method is expected to be rather poor in terms of the cost performance.
On the other hand, the method using the ATM exchange module connector has the problem in that it becomes necessary to provide a high speed and wide bandwidth (about 156 Mb/s) ATM path exclusively allocated for the transmission of the control data between each exchange module and the ATM exchange module connector, so that the utilization efficiency of the ATM paths can be quite low whenever the bandwidth required by the transmission of the control data is not so wide. This problem will be particularly severe in view of the aforementioned expected increase of the burstyness of the control data transmissions. In addition, the ATM exchange module connector will be quite large sized because of all these ATM paths to be provided, even when the size of the exchanger itself is not so large. Consequently, this method is also expected to have a rather poor cost performance as it demands rather large investment for the exchanger.
Furthermore, the problem mentioned above in relation to the previous method regarding the increase of the cost required for the control data transmission lines is also present in this method, and this can make this method even poorer in the cost performance.
Now, in such a conventional building block type exchanger, the setting of the capacity of the transmission path between any two exchange modules has been managed by the exchange modules autonomously, according to the time variation of the communication traffic between these two exchange modules, by making a negotiation between these two exchange modules about the capacity that can be allocated to that transmission path and then controlling the STM exchange module connector according to the agreement reached by the negotiation.
However, this manner of setting the capacity of the transmission path is associated with the following problems.
First, when such an autonomous management of the transmission path capacity by the exchange modules themselves is adopted, because the STM exchange module connector has a finite transmission capacity, there are cases in which the desired amount of the transmission capacity cannot be secured for some exchange modules because of the transmission capacities already used by the other exchange modules, in which cases the required set up quality such as a call loss rate cannot be satisfied for some exchange modules.
Secondly, in a case of setting up a new transmission path capacity, there are cases in which the required call set up quality cannot be satisfied during a period of time between the making of the request for the set up of the new transmission path capacity and the actual allocation of the requested new transmission path capacity.
Thirdly, when the available transmission capacity at the STM exchange module connector is little, a new transmission path capacity cannot be set up when it becomes necessary, even when there is a transmission path whose utilization efficiency is low, so that the utilization efficiency of the transmission capacity of the STM exchange module connector as a whole cannot be improved.
On the other hand, in such a conventional building block type exchanger, the set up or the release of the transmission path between any two exchange modules is made by either one of the following two methods.
(1) The exchange module connector makes the set up or the release by judging the appropriate timing for the set up or the release. PA1 (2) One of the exchange modules makes a request for the set up or the release to the exchange module connector by judging the appropriate timing for the set up or the release, and then the exchange module connector in turn makes a request for the set up or the release to the other one of the exchange modules.
However, these methods for the set up or the release of the transmission path are associated with the following problems.
Namely, in the method (1), it is necessary for the exchange module connector to be equipped with a function and a mechanism for the regular centralized monitoring of the utilization states of the transmission paths, as well as a function and a mechanism for carrying out the set up and the release of the transmission path and the associated set up data to be used between the transmission path and the exchange module. Here, the set up data must be updated every time the addition or the deletion of the exchange module is made in the exchanger, and this can lead to the decrease of the reliability due to the potential for making a mistake at a time of the updating of the set up data as well as to the complication of the operation required for the set up or the release. In addition, the reliability of the exchange module connector is also reduced because of the increase of its hardware required by the incorporation of a function and a mechanism for the regular centralized monitoring.
As for the method (2), it is also necessary for the exchange module connector to be equipped with the set up data to be used between the transmission path and the exchange module, so that the problem of the reliability mentioned above for the method (1) is also present. In addition, in this method (2), there are possibilities For two exchange modules to make the requests for the same set up or release simultaneously to the exchange module connector such that the exchange module connector carries out two equivalent set up or release operations in unnecessary duplication.